Metal oxycarbene

Metal-oxycarbene complexes react with Wittig ylides to provide a new route to vinyl ethers (Scheme 152). 

The straightforward access to O-glycosidic metal carbenes relies on the nucleophilic addition of an unprotected sugar alcohol to the electrophilic carbene carbon. In order to overcome problems arising from the inherent steric bulk of protected sugar nucleophiles the electrophilicity of metal oxycarbenes can be enhanced by 0-acylation. Acyloxycarbene complexes are temperature-sensitive intermediates but readily accessible from tetramethylammonium [acyl(pentacarbonyl)]metalates... 

A further synthetic approach to carbon-metal double bonds is based on the acid-catalyzed abstraction of alkoxy groups from a-alkoxyalkyl complexes [436 -439] (Figure 3.11). These carbene complex precursors can be prepared from alk-oxycarbene complexes (Fischer-type carbene complexes) either by reduction with borohydrides or alanates [23,55,63,104,439-445] or by addition of organolithium compounds (nucleophilic addition to the carbene carbon atom) [391,446-452]. 

The coordination of an oxophilic metal to the oxycarbene side chain offers the possibility to tune the properties and the reactivity of the carbene complex. A review on metalloxycarbene complexes has been published recently [88] among them boroxycarbene complexes have received the widest attention and have been applied to organic synthesis. Their reactivity strongly depends on the boron coordination sphere as demonstrated for dialkyl- and difluoroboroxycar-bene complexes. 

The insertion of CO into a C-Ln bond has also been demonstrated. After some unsuccessful attempts (8), the formation of a dihapto complex in the first stage of reaction and of a dimeric enedione-diolate final structure has been described (9). Such non-classical activation of CO, also observed in the case of actinides (10) as well as of some d elements like Zr (11), affords an oxycarbene lone pair completing the valence shell of the metal, and results in a new carbon-carbon bond formations. The research on CO chemistry and on the understanding of its fundamental aspects is of increasing importance in today s shifting situation to coal-based processes. 

Methyl trichloroacetate reacts with olefins in the presence of transition-metal catalysts to give mixtures of the corresponding butyrate and y-butyrol-actone. Substituted y-butyrolactones are produced in the radiation-induced addition of alcohols or the electrochemical reductive addition of acetone to a/3-unsaturated esters. Acetylenic Grignard reagents react with y-lactones to give products of double addition, whereas the analogous lithium salts react only once. The products observed in the pyrolysis of y-lactone toluene-/ -sulphonylhydrazone sodium salts can be accounted for by the intermediacy of an oxycarbene. ... 

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